Leuco vat dye preparations in granule form

ABSTRACT

Granules comprising as essential constituents a leuco vat dye, an alkali metal hydroxide and a mixture of from 10 to 100% by weight of an α-hydroxy-C 1  -C 4  -alkylsulfinic acid, one of its salts or derivatives and from 0 to 90% by weight of hydrosulfite as reductant, are prepared and used for dyeing cellulosic textile material.

Leuco vat dye preparations in granule form

The present invention relates to novel leuco vat dye preparations ingranule form comprising as essential constituents a leuco vat dye, analkali metal hydroxide and a mixture of from 10 to 100% by weight of anα-hydroxy-C₁ -C₄ -alkylsulfinic acid, one of its salts or derivativesand from 0 to 90% by weight of hydrosulfite as reductant.

The present invention further relates to the formation of these leucovat. dye preparations and to their use for dyeing cellulosic textilematerial.

Indigoid dyes (especially indigo itself and its bromine derivatives suchas Brilliant Indigo) and anthraquinonoid dyes (especiallynitroviolanthrone (C.I. Vat Green 9)) are well known vat dyes, which areused for dyeing cellulosic textile material.

For dyeing the water-insoluble vat dye first has to be reduced (vatted)to convert it into the water-soluble leuco form, which has affinity forthe fiber and which, after going onto the material to be dyed, isoxidized back to the water-insoluble vat dye pigment.

In existing dyeing processes, the vat dye is vatted in an alkalinemedium in a vessel upstream of the dyebath by addition of inorganicreductants such as hydrosulfite (sodium dithionite) and thiourea dioxideor else organic reductants such as hydroxyacetone. Additionalmachine-dependent portions of reductant are consumed during dyeing,since some of the leuco dye is oxidized by air contact in the airpassages and at the dyebath surface and has to be revatted, which is whysmall amounts of reductant also have to be added to the dyebath.

One disadvantage with the vatting of vat dye using the reductantsmentioned is the high contamination of the dyehouse wastewater withsulfate (from about 3500 to 5000 mg/l, measured in the wastewater of anindigo dyehouse) in the case of hydrosulfite or with oxygen-consumingsubstances (COD about 8000 mg/l, measured in the wastewater of an indigodyehouse) in the case of hydroxyacetone.

For indigo, there have also been described pulverulent solid to pastypreparations of the leuco form, which comprise polyhydroxy compounds,such as glycerol., and especially sugar derivatives, for examplemolasses, mixed with alkali or zinc dust as stabilizers (DE-C-200 914 or-235 047). Furthermore, GB-B-276 023 discloses the preparation ofpulverulent leuco dye preparations by heating a mixture of unreduced vatdye, glycol, alkali, hydrosulfite and sodium sulfate, drying in an openvessel and subsequent grinding.

The former preparations are effective in lowering the sulfate content ofthe dyehouse wastewater, but they are typically difficult to meter,since even the drying preparations tend to clump, and they are very slowto dissolve in the dyebath. With the latter preparations, the sulfatelevel is in fact increased, since these preparations, already comprisingthe sulfate from the reduction of the dye in the above-describedmixture, have additional sulfate added to them.

WO-A-94/23114 discloses a dyeing process wherein indigo is used in aprereduced form, as an aqueous, alkaline solution obtained fromcatalytic hydrogenation. This eliminates the wastewater loading withorganic substances, while the sulfate level is reduced to the amount ofhydrosulfite required for the air contact during the dyeing process.However, it is disadvantageous that the leuco indigo solution used isoxidation-sensitive and has to be handled and stored in the absence ofoxygen. In addition, the high water ballast of this solution is ahindrance as regards both storage and transportation.

Finally, WO-A-95/05421 describes leuco indigo preparations in granuleform comprising leuco indigo and alkali metal hydroxide as essentialcomponents. These granules likewise do not always show satisfactoryoxidation stability on prolonged storage in air, especially at highhumidity.

Furthermore, DE-A-1 9502 968,unpublished at the priority date of thepresent invention, describes hydrosulfite-stabilized leuco indigogranules.

It is an object of the present invention to provide leuco vat dyepreparations which do not have the disadvantages mentioned, which areoxidation-stable and which are advantageously useful for dyeing.

We have found that this object is achieved by leuco vat dye preparationsin granule form comprising as essential constituents a leuco vat dye, analkali metal hydroxide and a mixture of from 10 to 100% by weight of anα-hydroxy-C₁ -C₄ -alkylsulfinic acid, one of its salts or derivativesand from 0 to 90% by weight of hydrosulfite as reductant.

The present invention also provides a process for forming leuco vat dyepreparations, which comprises jointly concentrating an aqueous alkalinesolution of the leuco vat dye and a solution or suspension of thereductant in water and/or an organic solvent to dryness by thoroughmixing in the absence of oxygen.

The present invention finally provides a process for dyeing cellulosictextile material, which comprises dyeing with leuco vat dyepreparations.

The preparations of this invention generally comprise from 20 to 80% byweight, preferably from 50 to 70% by weight, of leuco vat dye, from 5 to55% by weight, preferably from 5 to 20% by weight, of reductant and from3 to 20% by weight, preferably from 4 to 10% by weight, of alkali metalhydroxide.

Suitable leuco vat dyes include not only the leuco forms ofanthraquinonoid vat dyes but also the leuco forms of indigoid vat dyes.

Preferred examples of suitable vat dyes are C.I. Vat Green 9 andespecially indigo and tetrabromoindigo (Brilliant Indigo).

The preparations of this invention include reductants comprisingα-hydroxy-C₁ -C₄ -alkylsulfinic acids (hereinafter referred to assulfinic acids for short), which are preferably comprised as salts, forexample as alkaline earth metal salts such as magnesium or calciumsalts, as zinc salts or especially as alkali metal salts such as sodiumor potassium salts, but can also be used in the form of otherderivatives, for example in the form of the reaction products withammonia such as trisodium nitrilotrimethylsulfinate.

Specifically, preferred examples are α-hydroxyethylsulfinic acid,α-hydroxypropylsulfinic acid and α-hydroxybutylsulfinic acid and theirsodium salts and especially hydroxymethylsulfinic acid and sodiumhydroxymethylsulfinate.

Mixtures of the sulfinic acids can also be used, of course. The novel,sulfinic acid leuco vat dye preparations are notable for particularstability. They are so stable in preparation and in storage that theycan not only survive long drying times but also be stored in hot moistair (fit for service in the tropics).

Novel preparations further comprising hydrosulfite as reductant are inaddition particularly readily and quickly soluble (spontaneouslysoluble). Any portions reoxidized in the course of handling or dyeingare immediately reduced back.

Mixtures of the sulfinic acids with hydrosulfite preferably have acomposition of from 85 to 15, particularly from 80 to 20, veryparticularly preferably from 60 to 40%, by weight of sulfinic acid andpreferably from 15 to 85,particularly preferably from 20 to 80,veryparticularly preferably from 40 to 60%, by weight of hydrosulfite.

Suitable alkali metal hydroxides are in particular potassium hydroxideand especially sodium hydroxide. Mixtures of alkali metal hydroxides canalso be used, of course.

The hydroxide converts the leuco vat dye into the readily soluble alkalimetal salts. leuco indigo, for example, forms the mono-and/or disalt,depending on the amount of alkali metal hydroxide.

The molar ratio between leuco vat dye and alkali metal hydroxide istherefore advantageously within the range from about 1:1 to1:10,preferably within the range from 1:1 up to the molar ratio requiredfor converting into the salt form all the free hydroxyl groups present.

In general, reductant quantities distinctly below 50% by weight,especially of not more than 25% by weight, based on the leuco vat dye,are sufficient to stabilize the leuco vat dye during storage of thegranule preparation (protection against reoxidation). A reductantquantity at the upper end of the range of the weight content mentionedabove for the preparations of this invention (from about 40 to 55% byweight), i.e., based on the leuco vat dye, of up to about 65% by weight,and sulfinic acid and hydrosulfite as a common reductant in which thehydrosulfite content is at least 50% by weight, will ensure that theportion of the vat dye inevitably reoxidized by air contact during thedyeing process is directly offset, so that no further reductant has thento be added to the dyebath. This reductant quantity can be variedslightly to optimize it to whichever dyeing machine is used.

For instance, in the case of the particularly preferred leuco indigopreparation, the amount of the preferred reductant mixture of sodiumhydroxymethylsulfinate and hydrosulfite required for stabilization isgenerally within the range from 5 to 30% by weight, in particular from10 to 20% by weight, based on leuco indigo, which corresponds to apreparation having a preferred composition of from 55 to 75% by weightof leuco indigo, from 10 to 20% by weight of the reductant mixture andfrom 10 to 20% by weight of alkali metal hydroxide.

The leuco vat dye preparations of this invention constitute readilymeterable, storage-stable (stable for several weeks at 50° C. in thepresence of air) dye preparations which, compared with the unreduced vatdye, contaminate the dyehouse wastewater to a significantly lesserextent and dissolve rapidly in the dyebath.

The essential requirement to stabilize the leuco vat dye againstreoxidation is its permanent and uniform thorough mixing with thereductant, which is ensured by the granule form of this invention (whichtypically comprises predominantly spherical particles having an averageparticle size of from in general 0.1 to 2 mm, preferably from 0.5 to 1.5mm).

In the case of the novel preparations which comprise hydrosulfite aswell as sulfinic acid, the proportion of hydrosulfite which only servesto compensate the reoxidation during dyeing can be subsequently mixed asa solid into the granules already sufficiently stabilized by thereductant mixture or the sulfinic acid alone. These mixtures areparticularly simple to adapt to the requirements of the particulardyeing machine.

The leuco indigo preparations of this invention can be advantageouslyformed by directly drying the aqueous solution obtained from thepreparation of leuco indigo or its derivatives by catalytichydrogenation after the catalyst has been separated off.

The catalytic hydrogenation itself can be carried out for example, ascommonly known, by reduction of an alkaline indigo paste (customarilyfrom 10 to 35% by weight of indigo, from 2 to 10% by weight of alkalimetal hydroxide) using Raney nickel as catalyst at a hydrogen pressureof from generally 2 to 10 bar and a temperature of from generally 60 to90° C.

The resulting leuco indigo solutions generally comprise from 10 to 35,preferably from 15 to 30, particularly preferably from 20 to 25%, byweight of leuco indigo.

Leuco vat dye starting solutions suitable for drying can be obtained ina similar manner in the case of anthraquinonoid vat dyes too.

In the novel process for forming leuco vat dye preparations, thealkaline leuco vat dye solutions are concentrated to dryness togetherwith a solution or suspension of the selected reductant in water and/oran organic solvent in the absence of oxygen, preferably afterinertization with a protective gas such as nitrogen, and by thoroughmixing.

The leuco vat dye solution and the reductant solution (or suspension) ispreferably mixed before or during the drying process, but the reductantcan also be added directly as solid to the leuco vat dye solution.

Suitable organic solvents for forming the reductant solution orsuspension include especially water-miscible solvents. Preferredexamples are C₁ -C₄ -alcohols, especially methanol, and also glycols,especially ethylene glycol, and glycol ethers, which can of course alsobe used in the form of mixtures.

A further interesting process variant comprises forming the sulfinicacid or its salt in situ in the course of the drying of the leuco indigosolution.

This can be done in a simple manner by adding hydrosulfite, thecorresponding C₁ -C₄ -aldehyde (mono- or dialdehyde) such asformaldehyde, glyoxal, acetaldehyde, propionaldehyde, malonaldehyde,butyraldehyde or succinaldehyde and alkali metal hydroxide to the leucoindigo solution in suitable mixing ratios.

The reaction taking place in the course of the drying will now beillustrated using the preferred hydroxymethylsulfinic acid as an exampleby means of the following reaction equation:

    Na.sub.2 S.sub.2 O.sub.4 +HCHO+NaOH→HOCH.sub.2 SO.sub.2 Na+Na.sub.2 SO.sub.3

If less than the stoichiometrically required amount of aldehyde isadded, then the preparation obtained after the drying will comprisesulfinic acid and hydrosulfite as reductant.

The product temperature required for the drying depends on whether ornot reduced pressure is employed and customarily ranges from 50 to 90°C. if it is employed and from 105 to 130° C. if it is not.

During the drying process, thorough mixing of the liquid phase and ofthe resulting solid has to be ensured.

Suitable large-scale apparatus therefore includes for example tumbledryers, paddle dryers and force-cleaned contact dryers, in each of whichthe drying is preferably carried out under reduced pressure (from about10 to 500 mbar). It is generally advisable to follow up the actualdrying with an afterdrying time of from about 1 to 2 h at the particulardrying temperature chosen. If necessary, an additional coarsecomminution can be carried inside or outside the drying apparatus.

The process of this invention can also be carried out to advantage in aspray dryer such as a spray tower and especially a fluidized-bed spraygranulator wherein the water is vaporized with a hot inert gas,preferably nitrogen. The dried product can in this case be obtaineddirectly in the desired particle size.

It is advisable to cool the dried product to generally <50° C. before itis discharged in order that any reoxidation in the hot state may beavoided.

The process of this invention provides a simple way of continuouslyproducing the novel leuco vat dye preparations in granule form. Ingeneral, the dried products have only a small unreduced vat dye content(about <4% by weight).

The leuco vat dye preparations of this invention are advantageouslyuseful for dyeing cellulosic textile material. The dyeings obtainedfully meet the requirements. The level of sulfur-containing inorganiccompounds in the wastewater is dramatically reduced compared with theuse of unreduced dyes. Depending on the composition chosen for the leucovat dye preparation, dyeing can also be carried out without furtheraddition of reductant to the dyebath.

EXAMPLES

A) Preparation of leuco indigo preparations according to this invention

Example 1

A solution of 21% by weight of leuco indigo (calculated as free acid),4% by weight of sodium hydroxide, 1.85% by weight of sodiumhydroxymethylsulfinate (99% pure), 1.85% by weight of hydrosulfite(calculated 100%) and 71.3% by weight of water was gradually introducedat 10 mbar into a nitrogen-inertized rotary evaporator heated to anoilbath temperature of 120° C. and concentrated to dryness at a speed of70 rpm. Following an afterdrying time of 2 h in the stated conditions,the temperature was reduced to <40° C.

The granules obtained following coarse comminution had an averageparticle size of 2 mm. Extraction with water in a Soxleth revealed awater-insoluble content (by-products and re-oxidized indigo) of <1% byweight. The granules comprised about 72.4% by weight of leuco indigo,4.3% by weight of sodium hydroxymethylsulfinate, 4.3% by weight ofhydrosulfite and 13.8% by weight of sodium hydroxide.

Example 2

A solution of 22% by weight of leuco indigo (calculated as free acid),7% by weight of sodium hydroxide, 4% by weight of hydrosulfite (87%pure) and 1.9% by weight of formaldehyde in the form of a 30% strengthaqueous solution was dried similarly to Example 1.

The granules obtained following coarse comminution had an averageparticle diameter of 1.5 mm. Extraction with water in a Soxleth revealeda water-insoluble content of <4% by weight. The granules comprised about64% by weight of leuco indigo, 7% by weight of sodiumhydroxymethylsulfinate, 7% by weight of sodium sulfite and 20% by weightof sodium hydroxide.

Example 3

A solution of 21% by weight of leuco indigo (calculated as free acid),4.5% by weight of sodium hydroxide and 74.5% by weight of water wasadmixed with 3.2% by weight of nitrilotrimethylsulfinic acid (calculated100% as 20% strength by weight aqueous solution) and then driedsimilarly to Example 1.

The granules obtained following coarse comminution had an averageparticle diameter of 1.5 mm. Extraction with water in a Soxleth revealeda water-insoluble content of about 4% by weight. The granules comprisedabout 73% by weight of leuco indigo, 11% by weight ofnitrilotrimethanesulfinic acid and 15% by weight of sodium hydroxide.

Example b 4

A solution of 22% by weight of leuco indigo (calculated as free acid),5% by weight of sodium hydroxide, 3.3% by weight of sodiumhydroxymethylsulfinate (99% pure) and 69.7% by weight of water was driedsimilarly to Example 1.

The granules obtained following coarse comminution had an averageparticle diameter of 1.5 mm. Extraction with water in a Soxleth revealeda water-insoluble content of <1% by weight. The granules comprised about71% by weight of leuco indigo, 10.5% by weight of sodiumhydroxymethylsulfinate and 16.5% by weight of sodium hydroxide. B)Dyeing with leuco indigo preparations according to this invention

Example 5

The dyeing was carried out in a customary indigo dyeing range with justone bath of 2000 l liquor capacity, fourfold dipping and oxidizing (4passes). The throughput of 4000 ends of metric count 12 raw cotton yarnwas 600 kg/h. The fixed indigo content was 1.8%.

Under these production conditions (standard), the leuco indigo granulesof Example 1 and, in a further run, the leuco indigo granules of Example2 were continuously sprinkled into the dyebath at 23.5 kg/h during the 8h duration of the experiment to maintain the previously establishedsteady-state (constant) conditions. Additional reductant and base onlyhad to be used in half the amount required for the standard dyeing withunvatted dye. The dyed yarn showed the same hue, depth of shade andfastness as a yarn dyed in a conventional manner under constant additionof

65 kg/h of leuco indigo solution 20% (calculated as free acid)

4 kg/h of 88% strength hydrosulfite

15 l /h of 38 ° Be sodium hydroxide solution.

We claim:
 1. Leuco vat indigo preparations in granule form comprising asessential constituents leuco indigo or a leuco form of an indigoderivative, an alkali metal hydroxide and a mixture of from 10 to 100%by weight of an α-hydroxy-C₁ -C₄ -alkylsulfinic acid, one of its saltsor derivatives and from 0 to 90% by weight of hydrosulfite as reductant.2. Preparations as claimed in claim 1, comprising from 20 to 80% byweight of leuco indigo, from 3 to 20% by weight of alkali metalhydroxide and from 5 to 55% by weight of reductant.
 3. Preparations asclaimed in claim 1, wherein the α-hydroxy-C₁ -C₄ -alkylsulfinic acid ishydroxymethylsulfinic acid or a salt thereof.
 4. A process for formingleuco indigo preparations as claimed in claim 1, which comprises jointlyconcentrating an aqueous alkaline solution of leuco indigo or the leucoform of a leuco indigo derivative and a solution or suspension of thereductant in water and/or an organic solvent to dryness by thoroughmixing in the absence of oxygen.
 5. A process as claimed in claim 4 inan inert gas atmosphere and/or under reduced pressure.
 6. A process asclaimed in claim 4, wherein the leuco indigo solution is mixed withhydrosulfite, a C₁ -C₄ -aldehyde, additional alkali metal hydroxide andwater and the α-hydroxy-C₁ -C₄ -alkylsulfinic acid is prepared in situin the course of the drying.
 7. A process as claimed in claim 4 in aspray dryer.
 8. A process for dyeing cellulosic textile material, whichcomprises dyeing with leuco indigo preparations as claimed in claim 1.